The present invention relates to smoke detector systems and, in particular, to a smoke detector system that has internal self-diagnostic capabilities and needs no recalibration upon replacement of its smoke intake canopy.
A photoelectric smoke detector system measures the ambient smoke conditions of a confined space and activates an alarm in response to the presence of unacceptably high amounts of smoke. This is accomplished by installing in a housing covered by a smoke intake canopy a light-emitting device (xe2x80x9cemitterxe2x80x9d) and a light sensor (xe2x80x9csensorxe2x80x9d) positioned in proximity to measure the amount of light transmitted between them.
A first type of smoke detector system positions the emitter and sensor so that their lines of sight are collinear. The presence of increasing amounts of smoke increases the attenuation of light passing between the emitter and the sensor. Whenever the amount of light striking the sensor drops below a minimum threshold, the system activates an alarm.
A second type of smoke detector system positions the emitter and sensor so that their lines of sight are offset at a sufficiently large angle that very little light propagating from the emitter directly strikes the sensor. The presence of increasing amounts of smoke increases the amount of light scattered toward and striking the sensor. Whenever the amount of light striking the sensor increases above a maximum threshold, the system activates an alarm.
Because they cooperate to measure the presence of light and determine whether it exceeds a threshold amount, the emitter and sensor need initial calibration and periodic testing to ensure their optical response characteristics are within the nominal limits specified. Currently available smoke detector systems suffer from the disadvantage of requiring periodic inspection of system hardware and manual adjustment of electrical components to carry out a calibration sequence.
The canopy covering the emitter and sensor is an important hardware component that has two competing functions to carry out. The canopy must act as an optical block for outside light but permit adequate smoke particle intake and flow into the interior of the canopy for interaction with the emitter and sensor. The canopy must also be constructed to prevent the entry of insects and dust, both of which can affect the optical response of the system and its ability to respond to a valid alarm condition. The interior of the canopy should be designed so that secondary reflections of light occurring within the canopy are either directed away from the sensor and out of the canopy or absorbed before they can reach the sensor.
An object of the invention is, therefore, to provide a smoke detector system that is capable of performing self-diagnostic functions to determine whether it is within its calibration limits and thereby to eliminate a need for periodic manual calibration testing.
Another object of the invention is to provide such a system that accepts a replacement smoke intake canopy without requiring recalibration.
A further object of the invention is to provide for such a system a replaceable smoke intake canopy that functions as an optical block for externally infiltrating and internally reflected light and that minimally impedes the flow of smoke particles to the emitter and sensor.
The present invention is a self-contained smoke detector system that has internal self-diagnostic capabilities and accepts a replacement smoke intake canopy without a need for recalibration. A preferred embodiment includes a light-emitting diode (xe2x80x9cLEDxe2x80x9d) as the emitter and a photodiode sensor. The LED and photodiode are positioned and shielded so that the absence of smoke results in the photodiode""s receiving virtually no light emitted by the LED and the presence of smoke results in the scattering of light emitted by the LED toward the photodiode.
The system includes a microprocessor-based self-diagnostic circuit that periodically checks the sensitivity of the optical sensor electronics to smoke obscuration level. There is a direct correlation between a change in the clean air voltage output of the photodiode and its sensitivity to the smoke obscuration level. Thus, by setting tolerance limits on the amount of change in voltage measured in clean air, the system can provide an indication of when it has become either under-sensitive or over-sensitive to the ambient smoke obscuration level.
The system samples the amount of smoke present by periodically energizing the LED and then determining the smoke obscuration level. An algorithm implemented in software stored in system memory determines whether for a time (such as 27 hours) the clean air voltage is outside established sensitivity tolerance limits. Upon determination of an under- or over-sensitivity condition, the system provides an indication that a problem exists with the optical sensor electronics.
The LED and photodiode reside in a compact housing having a replaceable smoke intake canopy of preferably cylindrical shape with a porous side surface. The canopy is specially designed with multiple pegs having multi-faceted surfaces. The pegs are angularly spaced about the periphery in the interior of the canopy to function as an optical block for external light infiltrating through the porous side surface of the canopy and to minimize spurious light reflections from the interior of the housing toward the photodiode. This permits the substitution of a replacement canopy of similar design without the need to recalibrate the optical sensor electronics previously calibrated during installation at the factory. The pegs are positioned and designed also to form a labyrinth of passageways that permit smoke to flow freely through the interior of the housing.
Additional objects and advantages of the present invention will be apparent from the following detailed description of a preferred embodiment thereof, which proceeds with reference to the accompanying drawings.